Back to EveryPatent.com
United States Patent |
5,135,047
|
Dobran
|
August 4, 1992
|
Furnace for high quality and superconducting bulk crystal growths
Abstract
The manufacturing of high temperature superconducting crystals and high
quality crystals in general requires a furnace design with very precise
process control to minimize the crystal defects. A furnace design of this
type can be realized by growing crystals in the cavity of the evaporator
section of a specially designed heat pipe, since the temperature of the
working fluid in the heat pipe, and thus in the cavity and crystal, can be
maintained very accurately by the control of the evaporating pressure of
the heat pipe's working fluid. The crystals are grown from the melt from
the bottom of a cooled and rotating crucible which is placed into the heat
pipe evaporator cavity. The growth of high temperature superconducting
crystals requires high furnace temperatures and an oxygen atmosphere. The
high temperature furnace condition is achieved with sodium and potasium
heat pipes. A double wall construction between the heat pipe and crystal
growing region allows for the maintenance of a vacuum condition between
these regions to prevent convective motions and potential safety problems
associated with furnace malfunction. The heat pipe condenser and radiation
heat sink zone of the evaporator cavity are cooled with fluids circulating
through the cooling jackets of the furnace. When the furnace is equipped
with a heat pipe working fluid pressure sensor, evaporator cavity and
crucible base temperature sensors, and heater and cooling fluid
controllers, the temperature control of the crystal growing environment
can be maintained within +-0.1.degree. C., for furnace operating
temperatures up to 1000.degree. C.
Inventors:
|
Dobran; Flavio (21 St. Broadway, Long Island City, NY 11106)
|
Assignee:
|
Dobran;Flavio (Long Island City, NY)
|
Appl. No.:
|
417326 |
Filed:
|
October 5, 1989 |
Current U.S. Class: |
165/48.1; 117/223; 117/954; 165/61; 165/70; 373/113; 373/158 |
Intern'l Class: |
C30B 029/06; C30B 035/00; B22D 027/04; F28D 015/02 |
Field of Search: |
165/61,64,48.1,70
422/248
156/616.4,616.41
373/113,158,154,165
|
References Cited
U.S. Patent Documents
2903495 | Sep., 1959 | Dickson et al. | 165/70.
|
3770047 | Nov., 1973 | Kirkpatrick et al. | 165/47.
|
3857990 | Dec., 1974 | Steininger et al. | 165/104.
|
4312700 | Jan., 1982 | Helmreich et al. | 422/248.
|
4980133 | Dec., 1990 | Koch | 156/616.
|
Foreign Patent Documents |
2213403 | Aug., 1989 | GB | 156/616.
|
Primary Examiner: Davis, Jr.; Albert W.
Claims
I claim:
1. An apparatus for growing single crystals, the apparatus comprising a
heat pipe with its evaporator forming an internal vertical cavity with an
opening at its lower end; a rotating and vertically moving crucible base
with provision for cooling on which a crucible with melt to be
crystallized is placed and inserted into the said cavity of the heat pipe;
a protecting wall between the crucible with melt and the said heat pipe
cavity to prevent possible reactions between different heat pipe working
fluids, gases in the crystal growing region and cooling fluids of the
furnace; and a radiation heat sink zone below the open end of the heat
pipe cavity.
2. The apparatus for growing crystals according to claim 1 wherein the
temperature gradient in the cavity of the heat pipe evaporator and in the
melt in crucible placed in the cavity is maintained by heating the
evaporator with electrical heating coils and cooling of the radiation heat
sink zone of the said cavity with a fluid; cooling of the condenser
section of the said heat pipe with a fluid; supplying the inside of heat
pipe evaporator with grooves and channels for efficient fluid transport in
the heat pipe; and employing a gas in the said heat pipe to form a
variable conductance heat pipe.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
1. M. E. Kirkpatrick, T. S. Piwonka, B. D. Marcus, Apparatus for
unidirectionally solidifying metals. U.S. Pat. No. 3,770,047,
November/1973.
2. J. Steininger, T. B. Reed, Heat pipe furnace. U.S. Pat. No. 3,857,990,
December/1974.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the art of crystal growing under controlled
temperature conditions for the purpose of producing crystals with minimum
number of defects.
2. Background Art
The discovery of high temperature superconductors above the liquid nitrogen
temperature of 77.degree. K. paves the way for numerous practical
applications. The Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 (123) and Bi.sub.2
Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10 (Bi2223) superconductors with critical
temperatures of 90.degree. and 110.degree. K., respectively, are only two
of an entire family of ceramics yielding a high temperature
superconductivity. Since the discovery of of the first high temperature
superconductor in 1987 above 77.degree. K., superconducting
polycrystalline pellets, oriented films, and small size bulk crystals have
been produced in laboratories. The films and bulk crystals have oriented
structures allowing for greater critical currents and magnetic fields than
the polycristalline superconducting materials.
The Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 is typical of the superconducting
perovskites and the inherent difficulties in manufacturing it are closely
related to other members of the family. As such, the crystal manufacturing
methods to be developed for this superconductor should be readily
extendable to other high temperature superconductors. The bulk material
processing problems of these superconductors are associated with
incongruent melting, melt nonstoichiometry, crystal growth anisotropy, and
the control of the processing environment such as oxygen pressure and
crucible type. Because of incongruent melting and anisotropy of grown
crystals, the most effective crystal processing methods appear to be
growth from the melt and a solution. The crystal growth of 123 and Bi2223
compounds from the melt involve growth from a CuO rich flux in ZrO.sub.2
and ThO.sub.2 crucibles by a slow cooling process where the flux is used
to reduce the melting point temperature. The critical crystal growth
conditions consist of the melt composition, temperature distribution and
cooling rate in the vicinity of the melt/crystal interface, temperature
distribution in the crystal, and crystal annealing time and thermal
cycling in an oxygen atmosphere. To achieve progressive crystallization of
the melt in the vicinity of the crystal/melt interface it is necessary to
remove the liberated latent heat and segregated solute from the interface.
These processes occur by the heat diffusion in the crystal and by
multicomponent mass diffusion and convection in the melt and solution,
with additional complications produced by the crystal growing apparatus
and radiation heat transport process. The growth of superconducting
crystals requires, therefore, a cooling of the crystal to remove the
latent heat and removal of the rejected solute from the interface region
into the bulk of the melt or solution. The techniques of single crystal
growth should, therefore, make provisions for removal of latent heat from
the growing crystal and allow for some type of convective mixing in the
melt or solution.
The production of high quality superconducting crystals thus requires the
minimization of apparatus vibrations and thermal oscillations in the melt
or solution and in the crystal. Thermal oscillations and constitutional
supercooling in the melt can produce defects leading to low critical
currents and magnetic field, or low quality crystals. Moreover, the
induced stresses in the crystal can produce crack nucleation and
deleterious effects on crystal properties. For these reasons, the
superconducting crystal growing apparatus should be designed with the
following characteristics: (1) minimization or absence of vibrations, (2)
with a control of thermal fluctuations or temperature in the melt and
crystal of better than 1.degree. C. from 600.degree.-1000.degree. C., and
(3) minimization of induced thermal stresses. To minimize the crack
nucleation in the neck of grown crystals and provide a constant diameter
shape, the crystal can be grown by simply placing the material to be
solidified in a cylindrical container and growing from a seed crystal at
the bottom, with the bottom of the container or crucible maintained below
the melting temperature. The elimination of vibrations can be achieved by
a furnace design involving no moving parts, and the heat zone should be
designed such as to impose on the solid and melt a thermal field whose
isotherms are parallel with the bottom of the crucible and have an upward
gradient.
BRIEF DESCRIPTION OF APPARATUS
A vertical solidification temperature gradient furnace for manufacturing of
high temperature bulk superconductors and high quality crystals in general
utilizes the evaporator cavity of a heat pipe in an oxygen environment.
The crystals are grown from the melt in a crucible which rests on a
crucible base that is cooled to remove the latent heat of solidification.
The working fluid in the heat pipe is heated with heating coils, and the
heat pipe condenser and evaporator cavity radiation heat sink zone are
cooled with fluids circulating through the cooling jackets of the furnace.
By appropriately cooling the heat pipe condenser, radiation heat sink zone
of the evaporator cavity, and the crucible base it is possible to
establish various temperature gradients within the heat pipe cavity and
maintain high temperature stability during the crystal growing process.
The high temperature insulation surrounding the heat pipe serves to
minimize the heat losses from the furnace. The double wall construction
between the heat pipe and crystal growing region allows for the
maintenance of a vacuum condition between these regions. This eliminates
the undesirable convective motions and provides for a great deal of safety
during the furnace operation. This safety is required to minimize the
potential problems associated with mixing of oxygen from the crystal
growing region and sodium or potasium working fluids in the heat pipe and
possibly with the cooling water when used as a coolant to cool the furnace
during the manufacturing of high temperature bulk superconductors. The
furnace wall is constructed from inconel and integrally welded in several
places to eliminate leaks. The crucible base is designed to provide
near-horizontal temperature gradients in the melt and it is cooled through
the integrally built cooling passages.
The furnace is also equipped with vacuum lines and a vacuum pump to
maintain vacuum between the heat pipe and crystal growing region. An
oxygen reservoir and pressure regulating valve are employed to establish
different oxygen pressure levels in the crystal growing environment, as
may be required to grow different types of superconducting crystals. The
temperature and temperature gradient control within .+-.0.1.degree. C. in
the crystal growing region of the furnace can be maintained by sensing the
pressure of the working fluid within the heat pipe, temperatures of the
evaporator cavity wall, temperature of the crucible base, and controlling
the heater power to heating coils and cooling fluid flow.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section view through the axis of a furnace having a
specially constructed heat pipe with crystals growing in the cavity region
of the heat pipe evaporator section.
FIG. 2 shows the crucible base detail of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
To obtain high quality crystal growth, the thermal field must be propagated
through the furnace in a desired fashion and rate at a high accuracy. Such
a vertical solidification thermal gradient apparatus can be constructed
with a temperature control of better than .+-.0.1.degree. C. and is shown
in a cross-sectional view in FIG. 1. The crystal growth 1 in this furnace
is accomplished in an oxygen atmosphere 2 that is situated in the cavity
region of the evaporator 5 of a high temperature specially constructed
heat pipe 3. To achieve the required crystal growth temperatures from
600.degree.-1000.degree. C., sodium can be used as the working fluid in
the heat pipe and can be evaporated with electrical resistance heating
coils 4 placed on the outer surface of the evaporator 5. The furnace
cavity for bulk superconductor growth is situated within the evaporator
section 5 of the heat pipe whose opening below provides for the radiant
energy escape and temperature decrease along the cavity. By properly
shaping and temperature calibrating the cavity, providing the heat pipe
with the condenser region 6, and cooling the radiation heat sink zone 7,
it is possible to establish the required temperature gradients in the
furnace or growing crystal 1. Moreover, by sensing the vapor pressure in
the heat pipe with a pressure transducer 8 it is possible to control the
heat pipe operating temperature and furnace to within .+-.0.1.degree. C.,
since the pressure can be monitored very accurately whereas the high
temperature cannot.
The heat pipe condenser 6 is radiatively cooled by the cooling jacket 9,
and a vacuum is maintained between the heat pipe 3, cooling jackets 9 and
13, and oxygen atmosphere 2. Thus, there is a double wall protection
between the sodium and oxygen atmospheres, insuring the absence of
undesirable convective motions in-between these walls and a safety
operation of the equipment. This then minimizes the possibility of sodium,
oxygen, and cooling fluids' leaks and interactions. The high temperature
zirconia insulation 11 and 12 provides for minimal heat losses from the
furnace to the environment. The cooling fluid jacket 13 is employed to
absorb the heat from the radiation heat sink zone 7 and provide for the
control of the thermal gradient in the crystal growing region 2.
The temperature gradient in the melt and crystal in crucible 14 and
extraction of the latent heat of solidification is also controlled by
cooling of the crucible base 15. The crucible base cooling is indicated in
FIGS. 1 and 2 by lines 16 and 17, whereas the condenser and radiation heat
sink zone cooling is indicated by lines 18 and 19. The fluid coolant lines
are equipped with flow control valves 20 and the cooling fluid may be
water. The vacuum in region 10 between the heat pipe and crystal growing
environment is maintained by a vacuum pump 21, which is also used to
evacuate the crystal growing region 2 before charging with oxygen from the
oxygen reservoir 22 through the pressure regulator 23. The furnace design
should employ the oxygen pressures from 0.1-1 MPa for growing different
types of ceramic superconductors. The flow control valves 24 are used for
isolating different parts of the furnace and control the vacuum conditions
in the apparatus.
The furnace walls 28 and heat pipe walls should be constructed from
inconel. The heat pipe can employ wick or groove structures in the
evaporator region 5. The grooves can be cut into the axial and radial
directions of the heat pipe walls on both sides of the evaporator (heater
and crystal growing sides). It is clear that this furnace design, with
crystal growing in the evaporator cavity of the heat pipe, can also be
used for manufacturing other types of crystals, such as gallium arsenide
if the oxygen atmosphere is replaced with an argon atmosphere and an
encapsulated (boric oxide) is used on top of the crucible melt to prevent
the loss of arsenic. In this situation the heat pipe should also employ a
noncondensable gas with pressure regulation to achieve the optimum GaAs
crystal growth conditions.
The crucible base 15 may be designed as shown in FIG. 1. Its top 25
consists of a high thermal conductivity material such as copper which is
cooled from below by a fluid which enters through the cooling passages 26
and exits through the passages 27. The leak-free condition between the
crucible base and furnace wall 28 can be maintained by the high
temperature seals such as O-rings 29 which are pressed tightly between the
walls of the plate 30 which is held secured to the furnace wall 28 by the
screws 31. The furnace can be manufactured in parts and, after assembled
with insulation, heat pipe, and heating coils, it can be joined together
by weldings 32 and 33. Welding insures the elimination of leaks and a
great deal of safety in the event of the system malfunction. The furnace
should also be equipped with a computer control which senses the pressure
from the pressure transducer 8, temperatures from the cavity wall 34, and
temperature from the crucible base 25. The proper control of the process
temperature in region 2 is then accomplished by controlling the heater
power to coils 4 and flow rates through the cooling jackets 9 and 13 and
crucible base 15. The crucible base may also employ a relative motion
control with respect to the furnace cavity. Such a control system can move
the crucible base vertically and rotate it for the purpose of more
accurately controlling the thermal gradient in the melt in crucible 14.
Top